Molybdenum disulfide is a known lubricant additive. Unfortunately, it has certain known disadvantages which are associated with the fact that it is insoluble in lubricating oils. Therefore, oil soluble molybdenum sulfide containing compounds have been proposed and investigated as lubricant additives. For example, in U.S. Pat. No. 2,951,040, an oil soluble molybdic xanthate is disclosed as being useful in lubricating compositions. Apparently, the molybdic xanthate decomposes under conditions of use to form an oil insoluble molybdenum sulfide on the metal surfaces being lubricated.
U.S. Pat. No. 4,013,571 discloses the use of certain thiosulfenyl xanthates in ashless lubricant compositions.
U.S. Pat. No. 4,259,254 discloses the use of xanthate containing molybdenum compounds in lubricating oil compositions.
U.S. Pat. No. 4,369,119 discloses an antioxidant additive for lubricating oils which is prepared by reacting an acidic molybdenum compound with a basic nitrogen compound and a sulfur compound and combining that product with an organic sulfur compound. In this regard, see also U.S. Pat. No. 4,395,343 and U.S. Pat. No. 4,402,840.
U.S. Pat. No. 4,474,673 discloses antifriction additives for lubricating oils which are prepared by reacting a sulfurized organic compound having an active hydrogen or potentially active hydrogen with molybdenum halide.
U.S. Pat. No. 4,497,719 discloses the use of metal salts of thiadiazole, such as molybdenum salts of thiadiazole as antiwear lube additives.
The foregoing patents are listed as representative of the many known molybdenum sulfur containing lubricant additives.
As is known in the art, some lubricant additives function as antiwear agents, some as antifriction agents and some as extreme pressure agents. Indeed, some additives may satisfy more than one of these functions. For example, metal dialkyl dithiophosphates represent a class of additives which are known to exhibit antioxidant and antiwear properties. The most commonly used additives of this class are the zinc dialkyl dithiophosphates. These compounds provide excellent oxidation resistance and exhibit superior antiwear properties. Unfortunately, they do not have the most desirable lubricity. Therefore, lubricating compositions containing these compounds also require the inclusion of antifriction agents. This leads to other problems in formulating effective lubricant compositions.
Additionally, extreme care must be exercised in combining various additives to assure both compatibility and effectiveness. For example, some antifriction agents affect the metal surfaces differently than the antiwear agents. If each type of additive is present in a lubricant composition, each may compete for the surface of the metal parts which are subject to lubrication. This can lead to a lubricant that is less effective than expected based on the properties of the individual additive components.
Thus, there still remains a need for improved lubricating oil additives that can be used with standard lubricating oils and that are compatible with other conventional components of the lubricating oil compositions.